Demonstrators surround a car, suspected to be a private taxi during a 24 hour taxi strike and protest in Madrid, Spain, Wednesday, June 11, 2014. The taxi drivers were protesting against unregulated competition from private companies, in particular, Uber, an international company that puts people in contact with each other to share cars or pay for short journeys in private vehicles within the city.

In a move that could speak for the legions of workers and business owners feeling outpaced by the digital economy, taxi drivers across several Europe's cities clogged traffic today as part of a mass protest against Uber, the popular car-sharing app.

London, Paris, Berlin, and Madrid felt the brunt of more than 30,000 disgruntled cab and limo drivers who argue Uber is unfairly moving in on one of the staple trades of the world's major cities.

"This is about an all out assault on our profession, our livelihoods," Max Small, a driver of one of London's black taxis for 34 years, tells Reuters. "These big companies are coming in, not playing by the rules."

Started in San Francisco in 2010, Uber lets people hail a cab through its free phone app. Uber screens its drivers and cars for safety and comfort and takes a portion of the fare charged to customers through their credit cards. The company has quickly grown to achieve a presence 128 cities in 37 countries around the world. Last week, the company was valued at $18.2 billion.

A key complaint among taxi drivers is that Uber operates essentially like a taxi service yet fails to adhere to local taxi rules and licensing regulations. For its part, Uber has refuted these allegations.

"What you are seeing today is an industry that has not faced competition for decades. Now finally we are seeing competition from companies such as Uber which is bringing choice to customers," says Pierre-Dimitri Gore-Coty, Uber Regional General Manager for Europe.

Reactions to the protest were varied. Some voiced support for the drivers by noting that the expertise provided by a professional driver could never be matched by an amateur Uber driver.

@KTHopkins I'd like to see the Uber drivers name any road in London simply by a description. Knowledge is like a damn degree.

More significantly, the protests serve as a touchstone for the debate around disruptive technologies that challenge traditional ways of doing business. In a blog post today, Neelie Kroes, vice president of the European Commission, said the protests were a way of prolonging the inevitable as opposed to adapting to change.

"Whether it is about cabs, accommodation, music, flights, the news or whatever[, the] fact is that digital technology is changing many aspects of our lives," she says in the post. "We cannot address these challenges by ignoring them, by going on strike, or by trying to ban these innovations out of existence."

Today's protests are by no means the first time Uber has faced controversy. Customers have long complained about the company's policy of "surge pricing," the practice of pricing Uber rides based on how in-demand its cars are at a given period. And Uber's policy of letting its drivers rate passengers on a scale of one to five stars has raised eyebrows, leaving many to wonder if this unfairly biases drivers against passengers with low scores.

Taxi drivers have also staged similar protests in the United States. But a recent article in The Washington Post points out that protests on such a wide scale as those in Europe are unlikely in the U.S. since American taxi drivers are often not unionized and are therefore competing against each other for fares.

Other ride-sharing apps include Lyft, Ridejoy, and SideCar, in addition to European ride-sharing apps such as Hailo, BlaBlaCar, and DJump.